In general, some image formation devices according to the method of electrophotography (e.g., electronic copier, laser beam printer, facsimile, etc.) are operated in such a manner that electrostatic latent images are formed on the surface of a photoconductor drum; images are developed on the drum by a toner; such developed images are transferred and fixed on a record paper to give a picture.
Hence, a method of forming the electrostatic latent images comprises in that after charging the whole surface of a photoconductor drum, the image of manuscript or a laser beam by the electric signal in a computer is exposed on the drum, and lost electric charge exposed by light results in obtaining the electrostatic latent images in the shape of manuscript or electric signal.
Meantime, the common methods of charging the surface of a photoconductor drum are being carried out by corona and contact methods but recently, there has been a trend toward the contact charging method as more preferable than corona, since the latter has encountered safety problems and excessive generation of ozone due to higher voltage (4.about.8 KV) of direct current used.
As illustrated in the attached FIG. 2 and FIG. 3, in such contact charging method a photoconductor drum (10), contacted with a charging roller (1), is rotated and then, a voltage between them is applied, thereby charging the surface of photoconductive layer (14) on the drum. The contact charging method is superior to the corona charging method in that no generation of ozone is observed since the former adopts a lower voltage (0.5.about.2 KV) than the latter.
Also, the above contact charging method uses an elastomer roller having good conductivity designed for better charging performance and active electric contact between the photoconductor drum (10) and charging roller (1).
However, if the conductivity of a charging roller is excessively large and electric resistance is too low, excessive current flow may inflict damage on the photoconductor drum or mechanical equipment. In case of defects such as pin hole on the surface of the photoconductor drum or damages of the fragment, excessive current is leaked out through the defective areas and the whole surface of the photoconductor drum contacted at that moment cannot be evenly charged. As a result, a black line will occur.
To overcome such occurrence of black line, the U.S. Pat. No. 5,126,913 disclosed a method of restricting the intensity of current by forming a high resistance layer at the surface of a conductive elastomer roller.
Nevertheless, if the surface resistance of a charging roller becomes too high, the intensity of current flowing at the surface becomes lowered and a lot of currents do not flow into some defected area, even if some defects such as pin hole exist at the surface of the photoconductor. Thus, the removal of black line may be available but when the surface of the photoconductor drum is charged, poor charging efficiency may cause a charging insufficiency.
In general, the resistivity of surface layer should be in the range of 10.sup.7 .about.10.sup.9 .OMEGA..multidot.cm so as to allow the appropriate conductivity to the surface layer but it has been reported that a very difficult technical problem exists in providing a roller surface layer with accurate resistivity, since such resistivity is in the scope of semi-conductivity.
The method of manufacturing the conventional charging roller comprises in that a common form of conductive powder (carbon black, graphite, metal powder, etc.) is dispersed in a binder (resin solution, etc.) to give a conductive paint, which functions to coat the surface of a conductive roller.
As for the conductive film, so formed, small numbers of conductive particles are mixed in the resin; in an accurate term, there are two different parts--extremely high resistance (resin) and extremely low resistance (conductive particles) and the distribution of conductive particles is thought not to be uniform at all surfaces.
In other words, the conventional conductive paint comprises in that a conductive powder is mixed with a binder such as resin and in case of the electric resistance at some areas where the conductive powder is crowded or dispersed in the resin, it is extremely higher or lower than standard value. In this respect, two local parts exist such as extremely high resistance and extremely low resistance.
If electric resistance of the surface layer using such conductive paint becomes extremely large charging insufficiency occurs. Further, when a part with extremely high resistance is contacted with the defective area of the photoconductor, black lines appear in the image recorded due to excessive current.
Therefore, it is very difficult to form an uniform surface layer having constant resistance values at the surface of a roller.